An aircraft lifting surface (see FIG. 1a) is usually structured by leading edges 11, torsion boxes 13, trailing edges 15 with control surfaces (flaps, elevators, rudders, etc.), a root joint 17 and tips 18.
A leading edge 11 is a structure responsible for keeping the aerodynamic surface with a torsion box surface, for supporting the static or cyclic structural loads involved and for protecting the torsion box from bird impacts. It is the part of the lifting surface that first contacts the air and the foremost edge of an airfoil section.
A known leading edge 11 (see FIG. 1b) comprises, on the one side, several ribs 21, called leading edge ribs (see FIGS. 2a, 2b), attached to the front spar 19 of the torsion box 13 and, on the other side, an aerodynamic profile 25 attached to the leading edge ribs 21 and to the flanges of the front spar 19 in order to keep the overall aerodynamic shape of the lifting surface. Leading edges further comprising additional elements like spars, vertical stiffeners and sandwich cores are also known in the art.
Nowadays, and particularly in the aeronautical industry, composite materials with an organic matrix and continuous fibers, especially CFRP (Carbon Fiber Reinforced Plastic) are widely used in a great variety of structural elements. Specifically, all the elements which make up the aforementioned leading edges 11 (leading edge ribs 21 and aerodynamic profile 25) can be manufactured using CFRP.
Typically, all structural elements forming an aircraft leading edge (aerodynamic profile, ribs, spars, vertical stiffeners) are manufactured separately and then joined by means of rivets with the aid of complicated tooling to achieve the necessary tolerances, which are given by the aerodynamic, assembly and structural requirements.
A well-known method for manufacturing said elements uses prepreg technology. In a first step, a flat lay-up of composite prepeg plies is prepared. Then the required shape is given to the element by means of a classical hot-forming process. After getting the required shape, the element is cured in a male or female tooling depending on the tolerances required and the overall manufacturing cost. Finally, after the curing cycle, the element contours are trimmed getting the final geometry, and then the element is inspected by an ultrasonic system to assure its quality.
The cost of a leading edge manufactured with said method is high because said steps shall be carried out independently for each element and a final assembly stage is needed.
The use of Resin Transfer Moulding (RTM) technology with dry fibers for manufacturing a leading edge in order to increase the level of integration and reduce the overall manufacturing costs is also known. Using this RTM technology, all dry laminates are formed to the final shape by means of classical hot-forming processes. Then, all formed laminates are co-injected together in a closed mould. The main benefits are linked to two facts; firstly, all reinforcements (like ribs, stiffeners and spars) are integrated without requiring additional subassembly activities; and secondly, only one curing, trimming and inspection process is required per element. Nevertheless, the tooling set required to build the whole element is complex making the demoulding process difficult and the overall manufacturing costs still high.